Write the formula of each compound. sodium tetrahydroxidozincate(II); hexaaquanickel(II) chloride…

Step 1. The zinc compound. Zinc is $+2$ and four hydroxide ions are $-4$. The complex ion has a charge of $-2$, so two $\text{Na}^{+}$ ions are needed: $\text{Na}_{2}[\text{Zn}(\text{OH})_{4}]$.

Step 2. The nickel compound. Water is neutral, so the complex ion has the charge of nickel, $+2$. Two $\text{Cl}^{-}$ ions balance it: $[\text{Ni}(\text{H}_{2}\text{O})_{6}]\text{Cl}_{2}$.

Step 3. The chromium compound. Chromium is $+3$ and six cyanide ions are $-6$. The complex ion has a charge of $-3$, so three $\text{K}^{+}$ ions are needed: $\text{K}_{3}[\text{Cr}(\text{CN})_{6}]$.